The present invention relates generally to disc drives. More particularly, the present invention relates to the writing of servo patterns in disc drives.
Disc drives include one or more magnetizable discs mounted on a spindle motor for rotation. Information is stored on the discs in a plurality of nominally concentric tracks. Heads mounted to an actuator are used for writing information to the tracks and for reading back stored information. The actuator is moved by a voice coil motor to position the heads with respect to the tracks. Head positioning information, or servo information, is prerecorded on at least one of the discs during the manufacturing process, typically using a servowriter.
Such servowriter systems typically include a controller that sends position commands to a stationary head for writing servo information to a disc. The disc on which servo information is to be written is rotated by a spindle motor beneath the head so that the head defines a circular track on the disc. Ideally, the tracks defined by the servowriter should be perfectly concentric. Unfortunately, various factors such as bearing tolerances, misalignment of the discs, mechanical resonance of the servo writing system, and others, tend to lead to errors in the location of the servo information. One of the factors contributing to irregularities is the vibration of the spindle motor, known as spindle non-repeatable runout (NRRO), of which a most significant component is the cage frequency. The presence of a large cage frequency may cause the disc to be radially displaced with respect to the head, causing the head to write a spiral instead of a circle. This is referred to as a track closure problem. Track squeeze errors may also occur. This is when neighboring tracks are too close to each other. Track squeeze is undesirable because it may generate data crosstalk between neighboring tracks or result in distorted servo information.
In a high-density disc drive where the track width is very small, track closure and track squeeze errors can have a detrimental impact on the performance of the disc drive. Therefore, it is important to eliminate the effects of spindle NRRO.
One conventional method reduces the written in repeatable runout (RRO) by starting the servo track writing operation at a selected phase of the cage frequency. However, as the servowriter must wait for that selected phase of the cage frequency before writing the next track, the servo write process takes a long time. In addition, the problems of track closure and track squeeze as caused by the cage frequency are not sufficiently addressed.
Some conventional servowriters have resorted to the use of a dedicated position sensor to measure the relative movement between the head and the disc, and then to compensate for the relative movement. The use of a dedicated position sensor can be costly, and in view of the increasing demand for cheaper yet better quality disc drives, some cheaper solution is needed to solve the problem of having ill-defined tracks resulting from spindle NRRO.
The present invention provides a solution to this and other problems, and offers other advantages over the prior art.
The present invention compensates for the effect of cage frequency when writing servo information, thus providing a solution to track closure and track squeeze problems. In one embodiment, a servowriter system reads back servo information from a track during a xe2x80x9cmeasurement interval.xe2x80x9d From the read back signals, a measured cage frequency value is extracted. As the head acts as a cage frequency sensor, in addition to writing servo information to the disc, the present invention is cheaper than conventional systems that require an additional laser position transducer dedicated to the measurement of cage frequency effects.
Based on the measured cage frequency, a predicted cage frequency is calculated. During a time period referred to as the servo write interval, the predicted cage frequency is fed, in a feed forward arrangement, to the servo commands controlling the position of the head. The head is thus made to follow the radial movement of the disc, and in this manner, cancel out the effects of the cage frequency. During the servo write interval, the servowriter system writes servo information to the disc.
At the end of the servo write interval, the process is repeated unless the last track for the disc has been written. The cage frequency compensation is switched off and the head reads back servo information from the last written track. This last written track is used as a reference track for the next measurement interval. As each subsequent predicted cage frequency should be more accurate than the previous predicted cage frequency, the servo write interval can be increased. Compared to the conventional method that requires synchronization with the cage frequency at every track, the present invention takes less time to complete the servo writing process as measurement of the cage frequency is only made intermittently.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.